1. Field of the Invention
The present invention relates to an automated abrader. In particular, the present invention relates to an apparatus for abrading tire tread compounds or other types of rubber compounds, to measure the wear rate, wear resistance and/or abrasion resistance of the compounds under simulated road wear conditions.
2. Discussion of the Related Art
Known methods and apparatus exist for abrading tire tread compounds or other types of rubber compounds. The apparatus generally include a moving abrasive surface. A sample of the compound to be abraded is engaged with the moving abrasive surface. The amount of material abraded from the sample is then measured to predict the wear rate, the wear resistance and/or the abrasion resistance of the tire tread compound of the sample under actual road conditions.
By way of example, some common laboratory abraders are the Lambourne abrader and the Akron angle abrader. The Lambourne abrader and the Akron angle abrader each test a solid rubber sample wheel by grinding the sample wheel against a grindstone wheel. In addition, each of these abraders uses a continuous slip between the grindstone wheel and the sample wheel. However, one problem with each of these laboratory abraders is that due to the continuous slip, the sample wheel surface can become significantly hotter than that of for example a radial tire which does not experience much slippage at the tire-to-road interface except during severe driving conditions such as braking, cornering or accelerating. For example, a 1987 American Chemical Society, Rubber Division Spring Meeting paper (No. 79) by Yoshi and Takeshita has shown that under continuous slip at 25% slip, the surface temperature of the solid rubber sample wheel rose to about 55.degree. C. (from 30.degree. C. ambient) in just one minute. This 25.degree. C. rise in surface temperature is not a condition that would be experienced by a radial tire under actual road wear conditions and thus leads to errors in the tested performance of the sample wheel.
Another known apparatus and method is the Pico test wherein a rotating razor blade is continuously engaged against the sample wheel to scratch the sample wheel. However, results based upon the Pico test do not correlate well with actual results under actual road conditions and therefore the Pico test is not accepted by the tire industry. Still another test is the DIN test, wherein a drum covered with sandpaper is continuously engaged with the sample wheel to wear the sample wheel. However, the DIN test suffers from the same infirmities as the Pico test.
Another problem with each of the above the known abrading methods and apparatus is that the abrasion data is taken only at one continuous wear rate level or at most a few continuous wear rate levels, in order to determine an average level of abrasion severity. However, it is known in the industry that the tread wear characteristic of tires under actual road wear conditions may vary dramatically at different levels of abrasion severity. As a result, the related art abrading apparatus and methods that abrade at only one level, or at most a few levels of abrasion severity, don't correlate very well with actual wear rate, and the like, of tires under actual road conditions.
Still another problem that results from the related art methods and apparatus is that a tire tread compound may indicate excellent abrasion performance at the tested level of abrasion, but when used in a tire and subjected to varying road wear conditions such as, for example, with a performance car, the tire will likely exhibit a high wear rate and/or develop an irregular wear pattern. Tires that exhibit irregular wear patterns tend to substantially decrease the life of the tire.
Accordingly, it is desirable to test the abrasion characteristics of tire tread compounds under a variety of conditions to simulate actual road wear performance. However, none of the known methods and apparatus have the capability to simulate the variety of conditions of road wear performance. Therefore, the industry has resorted to testing the tire tread compounds on road vehicles under a variety of conditions in order to predict actual road wear performance. However, this is a time-consuming and expensive procedure.
One known abrading apparatus of the related art is disclosed in U.S. Pat. No. 4,995,197, herein incorporated by reference. FIG. 1 illustrates a partial front view of the abrading apparatus 10 disclosed in U.S. Pat. No. 4,995,197. The apparatus includes a plurality of test stations 14 and a drive module 16 that drives each test station.
FIG. 2 illustrates a side view of one test station 14 of the abrading apparatus 10 of FIG. 1. Each test station includes a grindstone 20 that is keyed to a grindstone drive shaft 22. In addition, the test station includes a sample wheel 24 that is keyed to a sample wheel drive shaft 26 immediately above the grindstone. The sample wheel may include a steel core 25 (shown in phantom) and a layer of sample material compression molded over the steel core. The sample material may be, for example, a carbon black containing tire tread compound. The sample wheel is molded using techniques known to those of skill in the art.
As shown in FIG. 2, the sample wheel drive shaft 26 is journaled to a free end of a sample wheel frame 27. The sample wheel frame is pivoted about a second sample wheel drive shaft 28, to move the sample wheel 24 into and out of engagement with the grindstone 20. A normal force of the sample wheel against the grindstone is controlled by a counter-weight 29 that is suspended from the free end of the sample wheel frame by a cord 30. The test station 14 also includes a first sprocket 31, which is keyed to the first sample wheel drive shaft adjacent to the sample wheel. A second sprocket 32 is keyed to the second sample wheel drive shaft and is located in-line with the first sprocket. A sample wheel drive belt 34 is mounted over the first sprocket and the second sprocket to drive the sprockets and, therefore, to drive the sample wheel. As shown in FIG. 1, the second sample wheel drive shaft is coupled to the drive module 16, and thus the drive module drives the sample wheel 24.
Each test station 14 also includes a dust wheel frame 36, journaled at one end to the sample wheel frame 27 by a bearing support 38. A dust transfer wheel 40 is journaled at a free end of the dust wheel frame by a bearing support 42 and, as is shown in FIG. 2, is engageable with the sample wheel 24. The dust wheel frame is therefore pivoted about the bearing support to move the dust transfer wheel into and out of engagement with the sample wheel.
The test station further comprises a chalk stick arm 44, which is pivotally mounted at one end to the sample wheel frame 27, adjacent to the dust wheel frame 36. A chalk stick 46 is mounted at a free end of the chalk stick arm 44 by a bracket 47. A free end of the chalk stick is maintained in engagement with the dust transfer wheel 40, under the weight of the chalk stick and the chalk stick arm 44. A rubber band 54, having a width that is about a same width as a rubber core 50 of the dust transfer wheel 40, is fitted around a foam outer layer 52 of the dust transfer wheel. The rubber band is maintained in engagement with the both the sample wheel 24 and the bottom end of the chalk stick. The rubber band receives chalk dust from the chalk stick and, in turn, transfers the chalk dust to the outer surface of the sample wheel. The chalk dust is provided to control a surface condition of an interface between the sample wheel and the grindstone. In particular, the chalk dust is used to ensure wear of the sample under test.
A drawback of the dust transfer wheel assembly is that it does not uniformly dispense the chalk dust to the interface of the grindstone and the sample wheel. If there is insufficient chalk dust, an oily layer of degraded sample material builds up on the abrading surface of the grindstone 20, and decreases the rate of abrasion of the sample wheel 24. In contrast, if too much chalk dusk is provided, the dust can prevent effective contact between the sample wheel 24 and the grindstone 20, and thus decreases the rate of abrasion of the sample wheel 24. It is, therefore, desirable to be able to deliver a uniform amount of chalk dust to the grindstone and sample wheel. In addition, it may be desirable to vary a rate of the chalk dust delivered while ensuring that delivery of the chalk dust is uniform so as to simulate different road conditions, which is not possible with the related art apparatus.
In addition, it is known in the art that temperature is an important factor for rubber degradation. For example, rubber degradation increase with increasing temperature. Therefore, proper simulation of a realistic bulk temperature and surface temperature of the sample wheel is a requirement for a good lab abrasion test. The apparatus 10 includes several heaters 64, as shown in FIG. 1, which are mounted within the cabinet 12 to heat the interior of the cabinet. Thermocouples 65 are also mounted within the cabinet and are coupled to the heaters by wires 66, to control the heaters and maintain a desired temperature within the cabinet.
It is desirable to maintain a steady state temperature that is not too high within the cabinet 12 because the wear rate of sample wheels can change with running temperatures. However, as will be discussed in more detail below, the related art apparatus employs a rotational scheme of sample wheels 24 amongst the plurality of stations 14 and tests each sample wheel on a test station for only a few minutes. The rotational scheme prevents the sample wheels from being tested under a steady state bulk temperature and thus the related art apparatus relies upon the heaters and the thermocouples to provide the steady state temperature.
In addition, the related art steady state temperature is typically set to a temperature of, for example, 50.degree. C. However, the continuous wear rate level of the related art apparatus tends to heat the sample wheel to a temperature range such as, for example, 40.degree.-50.degree. C. above the steady state temp resulting in the bulk temperature of the sample wheels being too hot and in a wear rate of the sample wheels that does not correlate well with actual road wear performance.
The drive module 16 of the related art apparatus 10 also includes a gear that can be removed from the gear shaft and replaced by a different size gear to change a gear ratio of the drive module. The gear ratio controls a ratio of a velocity of the sample wheels 24 to a velocity of the grindstones 20, and is adjustable by manually selecting and replacing different size gears. The rate of abrasion of each sample wheel 24 in each test station 14 is achieved with the related art apparatus by controlling the ratio of the linear velocity of the sample wheels 24 to the linear velocity of the grindstone, which is also referred to as the slip value (S). The slip value S is defined as: ##EQU1## where V.sub.S is the linear velocity of the sample wheel 24, V.sub.G is the linear velocity of the grindstone, D.sub.S is the diameter of the sample wheel, D.sub.G is the diameter of the grindstone, and .OMEGA..sub.S /.OMEGA..sub.G is the gear ratio.
Referring to equation (1), the gear ratio .OMEGA..sub.S /.OMEGA..sub.G is usually chosen first and the slip S is then determined by the diameter ratio D.sub.S /D.sub.G. Thus, to change the slip ratio S, the gear of the drive module has to be manually changed. Thus, a drawback of the gear and drive shaft arrangement of the related art abrading apparatus 10 is that the apparatus only allows for discrete values of constant slip between the sample wheels and the grindstone. Another drawback of the related art apparatus is that the apparatus is labor intensive. For example, in order to vary the discrete values of slip, the apparatus requires the gear to be manually removed and replaced by a different gear.
In addition, the method of abrading samples of tire compounds with the related art apparatus includes, for example, runner sample wheels (runners) from each of three groups of sample wheels molded from a different tire tread compound. Additional sample wheels from each of the three groups are also used as dummies. The dummies are abraded during a conditioning step of the test, and are maintained within the apparatus while abrading the runners. The dummies are weighed when the runners are weighed during the test, and the weight gain or loss for the dummies is used to correct or adjust the measured weight gain or loss of the runners. The related art apparatus includes a balance accurate to about 0.1 mg that is used to weigh the sample wheels, and a laser micrometer that is used to measure diameters of the sample wheels.
More specifically, the runner sample wheels from each of the three groups are placed on a plurality of spindles in a random order, with six sample wheels per spindle. The dummy sample wheels are placed on separate spindles. A spindle of six sample wheels is tested on a group of six test stations. The runner sample wheels and the dummy sample wheels are conditioned prior to testing, for example, by heating the wheels to a steady state condition. The diameters of the runner sample wheels are measured as described above. The weight of each of the runner sample wheels and the dummy sample wheels are then measured and recorded to the nearest 0.1 mg. The apparatus is then preheated to a desired temperature and the spindles of sample wheels, including both the runners and the dummies, are heated inside the apparatus.
The runners from each spindle are then mounted on the respective sample wheel drive shaft 26 of the six test stations for each group of test stations in a predetermined order. The gear of the drive module is selected to provide a first slip value of, for example, approximately 7% and all of the runners are abraded for a predetermined duration of time such as, for example, for 20 minutes. The runners are then removed from the test stations and placed back on the spindle such that each runner when removed from the spindle of runners will be abraded in a new test station. When the runners are taken off of each test station 14 they are flipped over and placed on the spindle such that the direction of rotation of the runners is reversed. The runners are then mounted onto the respective test stations and abraded for a predetermined number of revolutions at the first slip ratio. The process is repeated six times such that each spindle of sample wheels is abraded on six different test stations. Accordingly, each sample wheel is abraded on each of the grindstones 20 of a group of test stations of the apparatus. After the six rotations between test stations, the runners are mounted on the spindles in the same order as prior to the initiation of the run.
Once all the sample wheels have been removed from the apparatus, they are cooled to room temperature, and the diameter of each runner, and the weights of all the sample wheels, including the runners and the dummies, are again measured and recorded as described above. The average weight loss or gain due to, for example, precipitation for all of the dummies of each compound is subtracted from or added to the weight loss or gain of the runners of each of the respective compounds.
The gear of the drive module is then changed to provide second and third slip values that are, for example, about 13% and about 21% respectively, and the above steps are repeated. The runners may be abraded at the 21% abrasion ratio, for example, for approximately 2 minutes. The 7%, 13% and 21% slip values are selected because they have been found to provide a correlation with road test results. The diameter of each runner, and the weight of both the runners and the dummies, are then measured and recorded, as described above. As can be seen from the above description the manual control of the related art apparatus yields a loss of test time and also requires constant attention by the operator.
Based on data resulting from the abrasion, a wear resistance of the tire tread compound is then computed. In particular, the wear rate (W) of each compound is calculated for each slip value. The wear rate (W) is an average volume loss per unit of travel (cc/cm) of the runners for each given compound. The volume loss (cc) is determined based on the measured weight loss of each runner (corrected based on the weight change of the dummies) and a density of the compound. The travel of each runner at each slip value is calculated by determining an average of each runner's diameter measurement before abrasion and a runner's diameter measurement after abrasion at the slip value. The average diameter measurement is then used to determine the average circumference of the runner at the slip value. The average circumference is then multiplied by the number of revolutions, to determine the travel of the runner at the slip value. The average wear rate at one slip value can then be calculated.
A drawback of the method and apparatus of the related art is that the test is performed for a limited number of constant slip ratios and thus there is no ability to simulate intermittent conditions that exist under actual driving conditions. Others drawbacks are that there is a variability of wear rate amongst the plurality of stations such as, for example, in the range from 30% to 50%. In addition, the complicated scheme of sample wheel rotation amongst all the stations requires time and attention of an operator and, prevents the sample wheels from being tested at the bulk temperature. Further, there is a need to remove and weigh the sample wheels in order to determine the wear rate for each slip value. Still further, there is a need to change the gear of the drive module in order to provide different constant slip ratios. Thus, the method and apparatus is not automated and requires stopping the test in order to make measurements. Moreover, because an average value of weight loss and an average value of travel are used, there are limitations to the accuracy in the predicted wear rate.
Accordingly, it is an object of the present invention to improve upon the methods and apparatus of the related art.